Potassium is the most abundant intracelluar cation, and is very important in maintaining physiological homeostasis. Potassium channels are present in almost all vertebrate cells, and the potassium influx through these channels is indispensable for maintaining hyperpolarized resting membrane potential.
Large conductance calcium activated potassium channels (also BK channels or maxi-K channels) are expressed especially in neurons and smooth muscle cells. Because both of the increase of intracellular calcium concentration and membrane depolarization can activate maxi-K channels, maxi-K channels have been thought to play a pivotal role in regulating voltage-dependent calcium influx. Increase in the intra-cellular calcium concentration mediates many processes such as release of neurotransmitters, contraction of smooth muscles, cell growth and death, and the like. Actually, the opening of maxi-K channels causes strong membrane hyperpolarization, and inhibits these calcium-induced responses thereby. Accordingly, by inhibiting various depolarization-mediated physiological responses, a substance having an activity of opening maxi-K channels is expected to have potential for the treatment of diseases such as cerebral infarction, subarachnoid hemorrhage, pollakiuria, urinary incontinence, and the like.
There have been various reports on a large conductance calcium-activated potassium channel opener, and examples of such channel opener are as follows; a pyrrole derivative disclosed in International Publication WO96/40634, a furan derivative disclosed in Japanese Provisional Patent Publication No. 2000-351773, and a nitrogen-containing 5-membered derivative in which the nitrogen atom is substituted by phenyl group or benzyl group disclosed in International Publication WO98/04135.
Also, a compound having a similar structure to the nitrogen-containing 5-membered heterocyclic compound which is an active ingredient of the present invention has been disclosed. For example, oxazole derivatives have been reported in Japanese Provisional Patent Publications No. 36614/1984, No. 152382/1984 and No. 172488/1984, but their uses are limited only to antihypolipidemic agent. Also, in Japanese Provisional Patent Publications No. 150591/1983, No. 34951/1985 and No. 54369/1988, imidazole derivatives have been reported but their uses are limited only to a cardiotonic, an antithrombosis, an antipyretic analgesic or an anti-inflammation agent.